Very little play in mine when locked in the open position (there is play, but the lock engages under a bit of tension in the hinge that resists being open wide enough for the lock to engage...as the hinge wears, I would expect more play to develop).
I am not sure if this is clear...if you open mine slowly, the lock will not engage. If it is slammed open (as happens during the deployment) it engages and locks up fairly solidly (for hinge/lock that has tiny parts like this).
I have little doubt that considerable play would develop over time, and the "lock" could easily be defeated if a person tried. That said, I never presumed that this knife was intended for hard use (or much use for that matter). To me, it is a neat gadget that could be carried and used in a pinch, but it would be a poor choice some that wants a do it all knife that they carry daily and use frequently.